1. Field of the Invention
The present invention relates to an antenna device and a portable radio communication device, and particularly to an antenna device and a portable radio communication device capable of reducing electromagnetic waves which are generated therefrom and are to be absorbed into a human body.
2. Description of Related Art
Recently, portable data transmitting/receiving devices capable of transmitting/receiving information by radio communication are significantly developed. Of the portable data transmitting/receiving devices, portable radio communication devices for use in the PHS (Personal Handyphone System) system are spreading rapidly.
The portable radio communication device usually has an antenna for transmitting/receiving signals. Actually, the portable radio communication device itself works as an antenna, and the main body of the portable radio communication device other than the antenna portion also generates electromagnetic waves. So, it is required that, of the electromagnetic waves generated from the portable radio communication device, those to be absorbed into a human body should be suppressed. Specifically, of the electromagnetic waves generated from the portable radio communication device in use, amount of electromagnetic waves to be absorbed into a specific portion of a human body, particularly a head portion, per unit-time per unit-weight is defined as local average SAR (Specific Absorption Rate), and the maximum value of the local average SAR is required to be not more than a prescribed value.
To reduce the maximum value of the local average SAR to be absorbed into a human body, a conductive plate of a predetermined shape may be used. In this case, the conductive plate has its one end connected to a ground conductor which works as an antenna to form a short circuit, and has its other end electrically released from the ground conductor. As a result, input impedance of the electrically released end becomes approximately infinite. At this time, high-frequency current flowing to the ground conductor is suppressed, and thus amount of radiation of the electromagnetic waves is reduced.
FIG. 1 shows a schematic view of a portable radio communication device 20, which can reduce the maximum value of the local average SAR. The portable radio communication device 20 includes a circuit board (not shown) necessary for performing radio communication, shield case 21 as a ground conductor which shields the circuit board, a conductive plate 22, an antenna feeding portion 23, and an antenna 24. The circuit board, shield case 21, and conductive plate 22 are enclosed by a housing (not shown) made of nonconductive material. The conductive plate 22 and shield case 21 are connected by a conductor 25 to form a short circuit.
Since the circuit board is shielded by the shield case 21, various circuits including a transmitting/receiving circuit for communicating with a base station which are mounted on the circuit board do not have bad effects upon each other, and also do not have bad effects upon the antenna 24 and other devices.
The transmitting/receiving circuit on the circuit board in the shield case 21 generates transmission signals of a predetermined signal form, and sends the transmission signals to the antenna 24 via the antenna feeding portion 23. Then, the antenna 24 transmits the transmission signals to the base station. The antenna 24 receives reception signals from the base station, and sends the reception signals to the transmitting/receiving circuit via the antenna feeding portion 23. Then, the transmitting/receiving circuit performs processing for the reception signals such as demodulating.
The antenna 24 is a rod antenna made of conductive wire materials, or a helical antenna made of conductive wire materials wound spirally. Otherwise, the antenna 24 may be an antenna of various types such as a stretch type antenna combining the rod antenna and helical antenna. When the portable radio communication device 20 performs radio communication, since the high-frequency current flows to the shield case 21 via the antenna feeding portion 23, not only the antenna 24 but also the shield case 21 as a ground conductor for the circuit board works as an antenna. That is, whole the portable radio communication device 20 works as an antenna.
When the portable radio communication device 20 is used, the user comes into contact with a speaker of the portable radio communication device 20. Since the shield case 21 as a ground conductor for the circuit board located behind the speaker also works as an antenna and radiates electromagnetic waves, there will be formed a portion where the value of the local average SAR becomes maximum around an ear of the user which comes into contact with the speaker, and this portion will be referred to as a hot spot.
The portable radio communication device 20 has the conductive plate 22 arranged such that the speaker (not shown) faces the conductive plate 22, and the conductive plate 22 and a front surface 21a of the shield case 21 are approximately parallel with each other with a slight interval therebetween. The interval between the conductive plate 22 and the front surface 21a of the shield case 21 depends on a radio communication frequency, and the portable radio communication device 20 can adjust the frequency bandwidth in accordance with the interval.
The conductive plate 22 has its one end along the longitudinal direction connected to the shield case 21 to form a short circuit via the conductor 25, and has its other end electrically released from the shield case 21. The length L3 between the short circuit forming end and the electrically released end is set to be a quarter of the radio communication frequency.
Accordingly, the impedance between the conductive plate 22 and the shield case 21 becomes close to zero at the short circuit forming end, while becoming approximately infinite at the electrically released end. Thus, the high-frequency current has difficulty in flowing from the antenna feeding portion 23 to the conductive plate 22 and the shield case 21.
As has been described, as an example to reduce the maximum value of the local average SAR to be absorbed into a human body, the portable radio communication device 20 mounts a conductive plate 22 thereto, and reduces the amount of radiation of the electromagnetic waves from the conductive plate 22 and shield case 21. Thus, the local average SAR of the hot spot can be reduced.
So as to form a short circuit surely between the conductive plate 22 and the shield case 21 of the portable radio communication device 20, the configuration of the short circuit forming end becomes complicated since, for example, the conductor 25 has to have elasticity. Thus, such configuration leads to high production cost and growth of weight caused by increase of the number of parts etc.
It is therefore an object of the present invention to overcome the above-mentioned drawbacks by providing an antenna device and a portable radio communication device which can reduce the maximum value of the local average SAR to be absorbed into a human body without forming a short circuit between the conductive plate and the ground conductor.
According to the present invention, there is provided an antenna device having an antenna element and a ground conductor which work as an antenna, in which the antenna element is fed via an antenna feeding portion and high-frequency current flows to the ground conductor via the antenna feeding portion, the antenna device including:
high-frequency current suppressing means being a conductive plate of a predetermined shape which has its both ends along one direction electrically opened from the ground conductor.
Furthermore, according to the present invention, there is provided a portable radio communication device which has an antenna device having an antenna element and a ground conductor which work as an antenna, in which the antenna element is fed via an antenna feeding portion and high-frequency current flows to the ground conductor via the antenna feeding portion,
wherein a circuit board for transmitting/receiving signals is shielded by the ground conductor, and
wherein the antenna device includes high-frequency current suppressing means being a conductive plate of a predetermined shape which has its both ends along one direction electrically opened from the ground conductor.
The longitudinal length of the conductive plate is an integer multiple of half a wavelength at the frequency of the radio communication.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.